The common objective of all disc drive manufacturers is to design smaller, quieter disc drives having greater storage capacity, while at the same time minimizing manufacturing costs.
As is well known, disc drives record and reproduce information on a constantly rotating disc or discs. Many disc drives, to maximize their capacity, are now made with multiple discs rotating on a single spindle. It is essential to the proper operation of the disc drive that the spindle motor which rotates the spindle and the discs be capable of smoothly and immediately starting the disc in rotation, bringing it up to a constant rotational speed, and maintaining that rotational speed under all operating conditions. In order to meet this objective, it is necessary to provide a spindle motor having a high starting torquej. This is achieved by providing a motor design wherein the minimum torque at any angular position is as high as possible. Further, during continuous speed operation, the motor must have a good fixed average torque, so that there is no lag in the rotation of the discs. These operating conditions are normally expressed as requiring a motor with a low torque ripple.
A critical problem in achieving low torque ripple is the existence of cogging torque. Cogging torque occurs when the lines separating the poles of the magnet line up with the slot openings between the coils, or windings of the stator. A number of prior art approaches have been taken to minimize the cogging torque and thereby provide a motor with a low torque ripple. One prior art approach is to provide a magnet with a skewed magnetic pattern. However, the result of such a design is a degradation in several of the other operating characteristics of the motor. Another approach commonly taken is to provide very thin slots between adjacent motor windings on the stator. However, this makes the motor extremely difficult to wind.